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Re: Dinosaurs and birds

On Wednesday, April 4, 2007, at 05:41 PM, dinoboygraphics@aol.com wrote:

And David M. asked about bats too: the problem with phrasing the question like this is that birds have many of their own adaptations to flight, and just because bats or pterosaurs have their own derived solution to a problem doesn't mean that a proto-bird that lacks the derived avian condition is magically exempted from a biomechanical problem.

Excellent point. However, on the flip side, the derived avian condition can largely hide what the minimum flight conditions are. For example, a common error is to assume that chickens represent a good minimum flight model, or that pigeons with cut supracoracoideus tendons demonstrate the basal pre-supracora. condition. The derived condition has become reliant on a supracoracoideus pulley; that does not mean that avian flight required one primitively. The pulley has distinct advantages, of course, but it may not have been biomechanically required basally.

As noted by Baier et al (Nature, 2007), the theropod glenoid capsule is shaped such that it is open on two ends (rather than being a true ball and socket joint). It's orientation in all maniraptorans (including avialans) is such that it cannot prevent dislocation of the humerus during the flight stroke unless their is tendonal and muscular opposition to the movement.

True. Though David Baier also has found that the supracoracoideus pulley does not stabilize the shoulder joint (pers. comm). Rather, the supracoracoideus destabilizes it further. The Acromocoracohumeral ligament is the primary mechanism for joint fidelity in derived avians. It may or may not have been in basal birds.

Niether of these derived solution that are unique to each group implies that a stem-group bird or early avialan that lacks the _avian_ derived condition can be exempted from these biomechanical problems.

True, but the fact that a given solution is important in derived avians doesn't mean that the basal condition required the same features, as well. It's a tough problem to crack. Like you said, the lack of given features in separate flying lineages is not particularly informative, but the presence in derived groups isn't always informative, either.

In other words, a "pulley-like supracoracoideus" is only necessary for flight if you lack other derived adaptations to solve the biomechanical problem.

The supracoracoideus pulley system is only necessary if a rapid upstroke is important. Since the supracoracoideus destabilizes the joint (along with pec. major), it falls almost entirely to ligamentous features to provide stability in modern birds. The supracoracoideus probably doesn't have much to do with stability of the shoulder in derived birds. You may have some additional data suggesting that it was more stabilizing primitively, though. I'm mostly just going on my chat with David Baier, since he's looked at the joint fidelity more than I have (though his result in that regard made sense to me).


--Mike H.